Oct 3, 2012 4:00pm EDT
candidates sweat a little bit. >> reporter: president obama looked relaxed this week. mitt romney, too. but he knows he has to change perceptions tonight. more voters now see romney negatively than see him positively. and that loses elections. >>. >> reporter: in virginia, romney's down by just two. with a lot on the line in past debates, romney has done well. >> going against romney is not a fun thing. he has the facts. he drills you, stays, tenacious. >> reporter: and sometimes aggressive. >> the idea that i'm anti-immigrant is repulsive. don't use a term like that. >> reporter: aides admit barack obama in debates can sound like a professorial know it all. >> i understand the broader point that senator clinton has been trying to make over the last several weeks. >> reporter: tonight's format is from 2008. moderator jim lehrer, 90 minutes, no breaks. with possible fireworks. >> i think you'll see both of them be more aggressive than either of their campaigns have predicted because the stakes are pretty high. >> reporter: the content could get even tighter, after tonight's clash.